Combatting Climate Change through Innovation: Urban Cooling

John Topping

Climate Institute
Washington DC.
Rudy Baum

Climate Institute
Washington DC.

By 2100, mean global temperatures are predicted to increase by 2-4°C as a result of human-caused global warming. Normally, discussions about climate change adaptation focus on how best to mitigate the secondary effects of these warmer temperatures—melting ice caps, rising sea levels, droughts and floods—but rarely is the increase in global temperatures seen as a threat to human health in and of itself. For the Arabian Peninsula, however, global climate change could push summertime temperatures to dangerous extremes. In cities situated on the Arabian Gulf, outdoor activity could become functionally impossible for large parts of the year: increasing heat and humidity around the Gulf could result in conditions under which the human body, no longer able to maintain a consistent internal temperature, can quickly become hyperthermic.

In order to adapt to this new, more extreme climate, cities across the region will have to adopt innovate new technologies in order to keep cool.

Albedo modification

One method of cooling cities already in use around the world is albedo modification—making urban spaces brighter in order to reflect more sunlight and reduce surface temperatures. Roofs are an obvious and already popular candidate for brightening. Roofs that have been painted white or treated with high-albedo materials stay up to 10°C cooler than an asphalt roof, and reduce ambient air temperatures by at least 1°C.

White roofs initiatives are only part of the solution, though. Urban canyons, which occur where streets are flanked by tall buildings on either side, create environments where, instead of escaping back into space, heat is emitted, reflected, and reabsorbed between the buildings. By treating buildings with high-albedo or reflective coatings, they can reflect more sunlight, which reduces the amount of heat that gets trapped between them. Researchers are currently investigating the effectiveness of retroreflective coatings, which reflect light directly back at its source instead of at nearby structures. By reflecting light away from other structures, retroreflective coatings have the potential to help reduce ambient temperatures within urban canyons.

Pavement is another area where significant gains in urban cooling can be made. Pavement, which is often quite dark, can make up over a fifth of a city’s surface area. By replacing asphalt with concrete, or treating asphalt with a high-albedo coating, street-level air temperature can be reduced by about 1°C. High-albedo pavement can have undesirable side effects such as road glare, but new materials are being developed that reflect strongly in the near-infrared range while still appearing relatively dark, allowing them to maintain cooler temperatures without blinding drivers and pedestrians. Another interesting possibility for reducing urban temperatures is the installation of pavement infused with phase change material (PCMs). Above a certain temperature, PCMs will melt, thus storing heat and keeping the pavement cool. When temperatures drop back down (like at night), the PCMs will release their stored energy and re-solidify.

Urban Planning

The geometry of a city also affects how efficiently it can be cooled, which can enhance the effects of albedo modification techniques such as cool roofs. Air often moves poorly through cities. By improving air circulation, cities can increase convective cooling, which in turn will lower ambient air temperatures and improve pedestrian comfort.

Aligning streets with the direction of prevailing winds can improve urban circulation and reduce peak daytime temperatures. It is also a good idea to incorporate street-level breezeways and open spaces into building designs. This allows maximum penetration of winds into urban spaces, which carry off heat and lower ambient air temperatures.

Thoughtful urban planning can also improve the effectiveness of albedo modification efforts. For example, while white roofs are excellent for cooling buildings, their effectiveness at reducing street-level temperatures decreases with increasing building height. By positioning taller structures downwind of white-roofed buildings, the cooler are can be deflected downward toward street-level.

Trees can also be used to improve thermal comfort in cities. Obviously, trees provide shade, but when planted in urban canyon environments, they can also help reduce ambient temperatures by disrupting the cycle of thermal reabsorption between buildings. Drought-resistant species, which can be watered with high-efficiency systems such as drip irrigation, can help provide shade and reduce ambient air temperatures without excessive demand for water.

Geoengineering

Around the Arabian Gulf, where the effects of excessive heat are predicted to be the most severe, geoengineering could potentially be used to complement urban albedo modification programs.

As water is quite dark, it absorbs a lot of thermal energy. Brightening the Gulf could help reduce ambient air temperatures, which would also decrease evaporation and help keep humidity down.

One option would be to build offshore structures that promote the formation of waves. Whitecaps have a significantly higher albedo than still water. If a large enough area can be brightened, local temperatures could potentially be reduced. The relatively shallowness of the Arabian Gulf means that the engineering challenges of such an endeavor would not be excessive.

Another option, which has been proposed by Climate Institute Senior Research Fellow Russell Seitz, involves injecting clouds of microscopic bubbles (microbubbles) into the waters near cities on the Gulf. These “hydrosols” are high reflective, and could significantly reduce local air temperatures. Although no system for mass distribution of microbubbles currently exists, it offers an intriguing possibility for future albedo modification.

Although the impact of geoengineering is potentially substantial, the technology to implement it is still at least several years away. It thus remains a largely theoretical option at the moment.

Conclusion

Although it is important that we take steps to mitigate the impact of climate change now, some of its effects are already locked in, and it's a good idea to take steps to adapt to them now. Improving the thermal environment of cities across the Arabian Peninsula will not only make them more livable in a warmer future, but by cooling them now, it will help reduce energy demand for air conditioning, thus decreasing fossil fuel emissions and helping to mitigate climate change. The benefits of innovative, sustainable urban design are clear in the present and in the future.

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